The north-south line of the magnetic compass usually does not point along the geographic meridian because:
a) the magnetic poles of the earth are located in a different position from the axial poles, thus causing an angle of error called "variation" on the magnetic compass and PA1 b) steel and other magnetic materials cause the lines of force of the earth's magnetism to deviate around a vessel carrying the compass and this results in angles of error on various courses called "deviation" effecting the magnetic compass.
Variation is uncorrected and the mariner allows for the variation. Deviation can be reduced or eliminated by distributing magnets and other soft iron correctors around the compass. Usually a small amount of residual deviation exists and, along with variation, creates an error on the compass.
The uncorrected variation and residual deviation are allowed for by the mariner and the course steered by the compass has an error and is usually different from the true course relative to the axial meridian. The mariner usually finds difficulty in the application of the variation and deviation and this invention (1) makes it easy to correct the variation and deviation effecting the compass, (2) makes it possible to correct the variation and deviation automatically and (3) enables the magnetic compass to show the true, or nearly true, direction relative to the axial meridian of the earth either manually or automatically.
Deviation is caused by many effects, all of which are of magnetic origin, except a part of deviation that can be caused by mechanical means. This part of deviation is caused if the lubber line, or heading, or steering marker is not fixed exactly along or parallel to the fore and aft or keel line of the vessel and it can result in an angle of error called "coefficient A". "Coefficient A" is a part of the deviation effecting the compass. Coefficient A causes an error in the compass equivalent to the angle that the lubber line is displaced from the keel line direction of the vessel.